Protein Preparation
¹⁵N,¹³C-labelling is commonly referred to as double labelling. The protein is produced by expression from bacteria which are grown on minimal medium supplemented with ¹⁵NH₄Cl and ¹³C-glucose.

Applications
This is a very common form of labelling and enables straight forward assignment of both the backbone and side-chain ¹H, ¹³C and ¹⁵N atoms using triple-resonance spectra. A high proportion of these assignments are required if you wish to calculate the structure of your protein accurately.

Reference:
L.P. McIntosh and F.W. Dahlquist (1990) Quart. Rev. Biophys. 23 1-38. (Link to Article)